Switching system



Jan 29, 1946. G, .1. MAKI v2,393,645

SWITCHING SYSTEM Filed Oct. 27, 1943 TELeseapH 4pm/arms o v /N VEN To@to such a difference.

Patented Jan. 29, 1946 SWITCRIING SYSTEM George J. Maki, United StatesArmy, Elmhurst, Ill.

Application October 27, 1943, Serial No. 507,802

(Granted under the act of March 3, 14883, as amended April 30, 1928; 3700. G. 757) 7 Claims.

The invention described herein may be manufactured and used by or forthe Government for governmental purposes, without the payment to me ofany royalty thereon.

This invention relates to improvements in radio signalling devicesadapted to be used in connection with telegraph, teletype', facsimileapparatus and the like.

It is an object of this yinvention to provide a device for telegraphicsignalling by using a radio frequency carrier. Intelligence istransmitted by causing the mark and space sense of signals so radiatedto be slightly different in frequency, such as a difference of 850cycles. That is, the mark (or the'space) signal may have a radiofrequency of 2,000,000 cycles per second, and the "space (or mark)signal may have a frequency of 2,000,850 cycles per second. The receivedsignal, when heterodyned in a suitable receiver, is caused to produceaudio frequencies 850,l cycles apart. These audio frequency signals canbe readily separated by means of band pass filters.

The advantage of using two-frequency signalling occurs at the receiver.Noise frequencies are received and passed through the receiving system,including the selective band pass filters, with substantially equalfacility. These noise voltages act with equal force upon the transducer,but in an opposite sense. This results in the cancellation of theeffects of noise frequencies to a large extent. The heterodynefrequencies present in either the mark or space filter at any instant.

The above and other objects are attained by the novel circuit andarrangement of elements hereinafter described and illustrated in theaccompanying drawing, forming a part hereof, and in which thel ngureshows a schematic diagram of a signalling system.

Referring to the drawing, the system is shown to include a pair ofsuitable generators i and of radio frequency energy, their respectivefren quencles being nearly but notl exactly the same. For example, thefrequencies of the generators may differ by 850 cycles per second.However, it should be noted that the system is not restricted The choiceof the difference in frequencies depends upon the extent of the radiospectrum to be employed, and also the capability oi' selective filtersof practical design to distinguish between the two radio frequencies.

Generators 1 and 8 are caused to excite am plier tubes i and 2, whichare of the pentode type. The cathodes of these tubes are connected toground"; and the screen grids are connected to a common point l30 on abattery 4 so as to provide a positive screen grid potential of suitablevalue with respect to the cathodes.

The plates of tubes l and 2 are connected together at a common point toa tuned circuit comprising a coil Il and a condenser I6. The resonantfrequency of this tuned circuit is midway between the frequencies ofgenerators 'i and 0. Positive voltage from battery` t is fed throughcoil I1 to the plates of tubes l and 2 to provide proper operation.

With xed values of grid potential and plate potential, a characteristicof a pentode tube is that when the suppressor grid voltage is madenegative with respect to the cathode, the space current will be reduced;and the space current may be reduced to zero by the application ofsumcient negative potentia1 on the suppressor grid. Likewise, a positivepotential on the suppressor grid will cause the space current toincrease and continue to do so until the point where saturation ofemission is reached, when no further increase in current will occur.Pentode tubes are commonly operated with zero' potential betweensuppressor grid and cathode.

In transmitting a signal, it is desired to select either generator l or0, whichever produces the desired frequency; and to block; out orprevent transmission of voltage through the tube connected to thegenerator producing the undesired frequency. The system depends upon itsability to transmit either freqency produced by the generators 'l and ilat will, but never -both frequencies simultaneously. To effect thisfunction, the suppressor grid of tube l is connected throughresistance.22 to junction C of a Wheatstone bridge, and suppressor gridof tube 2 is connected through resistance 23 to junction D of thebridge.

Arms il, l0, and ll have resistances of equal value. If the fourtharm-0f the bridge, composed of resistors l2 and I3, were the same asarms 9,

l0, and ll, the battery 5 connected across the other junction points Aand Bwould produce no potential difference across points C and D, sincethe bridge wouldbe balanced. `Common usage designates a condition ofbalance to exist when the potential across points C and D is zero.However, in this application of the bridge, an unbalanced condition isnecessary; and lthis'. can be effected by causing resistors l2 and i3 tovary in value either greater or less than the values of arms 9, l0, andil. 1

Assume point A to be negative and point B to be positive, with respectto each other. If resistors l2 and l It are caused to be greater invalue than any one of the other three arms, point C will be positive andpoint D will be negative with respect to each other. If resistors I2 andI3 are caused to be less than any one of the other arms, pointD will bepositive and point C will be negative with respect to each other. 'I'husby varying the value of resistors I2 and I3 such that their sum isgreater or less than any of the other arms, the potential differencebetween C and D can be caused to reverse. The current caused to ilow dueto this potential diierence will ilow from positive to negative ineither case, but in one instance when C is positive the current willflow from C to D, and when D is positive, the current will flow from Dto C.

It is seen that potentials developed acorss resistances I4 and I5 due tocurrent iiow from C to D, or vice versa, are impressed upon suppressorgrids of tubes I and 2. For example, assume current to be owing from Cto D. Point C, therefore, is positive with respect to ground, causingsuppressor grid of tube I to be positive with respect to ground which inturn enables tube I to function. While current continues to ow from C toD, point D is negative with respect to ground, causing suppressor gridof tube 2 to be negative with respect to the cathode of tube 2, thecathode being connected to ground, and preventing the functioning oftube2. With suitable choice of battery voltage 5 and resistors I4 .andI5, this negative potential can be made to block or prevent transmissionof voltage developed by generator 8 through tube 2. Likewise, thereverse situation obtains when resistors I2 and I3- are less than any ofthe other arms, causing D to be positive and C to be negative withrespect to each other.

In practice, resistor I2 is made as near to zero as practicable bymaking a direct connection and not inserting any resistance. Resistor I3is made as near infinity as possible by omitting any connection.Contacts of a relay @connect across resistor I3, the relay beingactuated by impulses created by a telegraph apparatus or the like. Whencontacts of relay 6 are closed, the resistance of arm BD is less than ADor BC. When contacts of relay 6 are open, arm BD has a greaterresistance than that of any of the other arms of the bridge.

It will be noted Ithat the transition time between reversals of currentthrough CD by virture of operation of relay 6 is substantially zero.Either one or the other situation exists, but not both or neither at anygiven instant.

As mentioned above, pentode tubes are usually operated with suppressorpotential positive with respect to the cathode. To limit the positivepotential to a small value, rectifier tube 3 is connected so that one ofeach of the plates of tube 3 is connected to a suppressor grid. Whenpoint C is positive with respect to ground, tube 3 conducts currentthrough the plate connected to suppressor grid of tube I. The currentthus caused to flow develops a voltge drop across 22. The greaterportion of the potential across IH thus appears across 22 and only asmall portion appears across tube 3 connected to suppressor' grid oftube I. On the other hand, tube. 3 does not conduct when D is negativewith'respect to ground, because current can only ow through the tubewhen the plate is positive with respect to the cathode. As, no currentis flowing through resistance 23, no voltage drop is developed acrossit. The full voltage developed across resistance I5 therefore isimpressed on suppressor grid of tube 2 with proper choice in the voltageof battery 4v and resistors Il and I5, and this potential can be causedto block oil tube 2. The reverse situation exists when point D ispositive with respect to ground.

The principal advantage of this system is a zero transition time betweenselection of generators 1 and 8, resulting in no interruption oi theradiated radio frequency carrier, but merely a shift in its frequency.Since there is no carrier interruption while signalling, the carrier canbe modulated by the amplitude method. The depth of modulation may not becompleted, however, as some carrier must be radiated to effect thesignalling described herein.

The speed of signalling is limited only by the external means such asthe operation of the apparatus controlling relay 6, the transitionsfollowing the operation of the contacts of relay 6. However, the systemis not restricted to a mechanical device such as relay Ii, but there maybe substituted for the relay any electronic device capable of changingthe resistance due to signalling impulses being impressed thereon.

The system is adaptable to any signalling system employing block orsquare Wave signal units. such as Morse telegraph, teletype, orfacsimile transmission. y

The current delivered by battery 4 is constant and has no component ofsignalling frequency due to the fact that at every instant either tube Ior tube 2 is functioning, but never both tubes together or neither.,Space currents of tubes I and 2 are adjusted to be equal, and also isthe value of voltages delivered by generators 1 and 8. Properly tunedand adjusted, current in the load I9 isA the same for either frequency.The plate voltage to each oscillator tube must be the same..

It will be seen that there has been provided a simple and effectivesystem for providing instantaneous switching between two signals of'different radio frequencies, and especially adapted for use inconnection with telegraph, teletype and facsimile apparatus.

The above description is to be considered as descriptive and notlimitative of the invention of which modifications may be made Withoutdeparting from the spirit and scope of the invention as set forth in theappended claims.

The invention having been described, what is claimed is: f

1. In a device for telegraphic signalling by using a radio frequencycarrier, a rst electron tube connected to a generator of a radiofrequency, a second electron tube connected to a generator of a slightlydiii'erent radio frequency, a Wheatstone bridge connected to the tubesto control the operation thereof, said bridge having one arm of variableresistance, and means controlled by the vopening or closing of a switchfor changing the resistance of said arm to change the grid bias of saidtubes so that one tube will be in operation and the other tubeinoperative.

2. In a device for telegraphic signalling by using a radio frequencycarrier, two generators of different radio frequencies, each of saidgenerators having an electron tube connected thereto, a Wheatstonebridge having three arms of equal resistance and a fourth arm vhaving aportion of very high resistance in series with a very low resistance, arelay controlled by a telegraph apparatus, said relay when closed beingconnected to the bridge to short circuit the very high resistance andwhen open to connect the very high resistance and very low resistance inseries, a pair of spaced resistances connected in series betweendiagonal junction points of the bridge, a connection to ground betweenthe pair of spaced resistances, one of the junction points beingconnected to one of .the electron tubes and the other junction pointconnected to the other electron tube, and means to regulate the voltagesapplied to the electron tubes.

Y3. In a device for telegraphic signalling by using a radio frequencycarrier, two generators ofv diiferent radio frequencies, each of saidgenerators having an electron tube connected thereto, each of saidelectron tubes having a suppressor grid, a Wheatstone bridge .havingthree arms of equal resistance and a fourth arm having a portion of veryhigh resistance in series with a very low resistance, a relay controlledby a telegraph apparatus, said relay when closed being connected to thebridge to short circuit the very high resistance, and when open toconnect the very high resistance and the very low resistance in series,a pair of spaced resistances connected in series between diagonaljunction points of thc bridge, a connection to groundbetween the pairvof spaced resistances, one of said junction points being connected tothe suppressor grid of one of the electron tubes and the other junctionpoint connected to the suppressor grid of the other electron tube, and arectier tube associated with the bridge to regulate the voltages appliedtothe suppressor grids.

4. In a device for telegraphic signalling by.

using a radio frequency carrier, two generators of different radiofrequencies, a first electron tube connected to one of the generatorsand having a. suppressor grid, a second electron tube connected to theother generator and having a suppressor grid, a Wheatstonebridge havingthree arms of equal resistance and a fourth arm having a portion of veryhigh resistance in series with a portion of very low resistancel a relaycontrolled by a telegraph apparatus, said relay being connected to thebridge to short circuit the very high resistance when closed and placingsaid very high resistance and very low resistance in series when open, aline having spaced resistances in series connected between diagonaljunction points of the bridge, the line being grounded at a pointbetween said resistances, one end of the line being connected to thesuppressor grid of the iirst tube and the other end of the line beingconnected to the suppressor grid of the other tube, and a rectifier tubeassociated with the bridge to regulate the voltages applied to thesuppressor grids.

5. In a device for telegraphic signalling by using a radio frequencycarrier, two generators of different radio frequencies, each of saidgenerators having an electron tube connected thereto, a Wheatstonebridge having three arms of high resistance and when open to connect thevery high resistance and the very low resistance in series, andconnections from points on the bridge to said tubes so that When onetube is operating the other tube is inoperative.

6. A device for continuous wave signalling, com'prising: first frequencygenerator means in cluding a tube having cathode, grid and plateelements; second frequency generator means including a tube havingcathode, grid and plate elements, the frequencies of said generatormeans being different but not substantially so; a Wheatstone bridgehaving three4 arms of fixed impedance and a fourth arm of variableimpedance; control means for varying said variable impedance; a directcurrent source connected to two opposite corners of said bridge toenergize it; a connection between the third corner of said bridge and anelement of said rst generator means tube; and a connection between thefourth corner of said bridge vand an element of said second generatormeans tube, so that when one tube is operating the other tube isinoperative.

7. A device for continuous Vwave signalling, comprising: first frequencygenerator means including a tube having cathode, grid and plateelements; second frequency generator means in` cluding a tube havingcathode, grid and plate elements, the frequencies of said generatormeans being different but not substantially so; a Wheatstone bridgehaving three arms of vfixed resistance and a fourth arm having aresistance higher than that of its associated arm; switch means adaptedto be closed to short at least suilicient of the resistance of thefourth arm to reduce the resistance of that arm below that of itsassociated arm; a direct current source connected to two oppositecorners of said bridge to energize it; resistance means connectedbetween the other two opposite corners of said bridge; a connectionbetween one of said other two opposite corners and an element f saidfirst generator means tube; a connection between the other of said othertwo opposite corners and an element of said second generator means tube;and a, common return from thecathodes of both of said tubes to anintermediate point on said resistance means, so that when one tube isoperating the other tube is inoperative.

GEORGE J. MAKI.

